A spectacle lens having a portion in which power continuously changes (so-called corridor) is known. Such a spectacle lens also called a “progressive power lens”, which has different types, for example, a progressive multifocal lens which includes a distance portion and a near portion, and a single vision lens of which power on the lens surface changes as departing from a region to view an object at a predetermined distance.
For example, in a progressive multifocal lens, a curve called the main line of sight or the meridian (hereafter called “main line of sight”), which is a reference when the power changes continuously, is set.
The main line of sight in this description refers to a line formed by points through which the line of sight passes in the spectacle lens when the user, wearing the spectacle lens, moves the line of sight from above to below. This main line of sight is the base when the spectacle lens is designed.
For example, the main line of sight is indicated in the spectacle lens in FIG. 1 of PTL 1. In PTL 2, it is suggested to determine the shape of the main line of sight considering various conditions on individual data of the specific user of the spectacle lens.